Here is my analysis of the sensors. Notice that each of the markings on the focusing screen must correspond with at least two lines of pixels. It also appears that with a "normal precision" array of two lines of pixels, the two lines of pixels are about as far apart as the length of a pixel line.

But the central sensor uses eight pixel lines (four arrays). The blue-coded arrays represent the "normal precision" two lines in each array, with both a vertical and a horizontal array.

The f2.8 "high precision" mode involves an entirely separate set of two arrays (red and green). I don't know if these come into play in addition to the f5.6 set or in place of them. Either would increase precision, as the pixel lines of the horizontally oriented array (red) spreads the lines twice as far apart, which in itself would increase accuracy.

Aha, Canon has recently published a 20d white paper which confirms a couple of points of information to the focusing picture.

1. As we had deduced earlier, the change from normal precision mode to high precision mode is, indeed, a switch function executed when a lens of f2.8 or faster is mounted. From what Chuck Westfall says in the latest Tech Tips on the Digital Journalist website, the camera interrogates the lens for this information even if the lens is mounted while the camera is "asleep"--something else we'd already deduced.

2. Also as we had deduced, the central focusing point contains a total of eight pixel segments.

a. Two pixel segments in line with each other comprise an array. There are two horizontally oriented arrays (detecting vertical details in the scene) and two vertically oriented arrays (detecting horizontal details in the scene).

b. A horizontally oriented array operates with a vertically oriented array as a "cross-type" sensor, so there are two "cross type" sensors represented by the marked square in the center of the focusing screen.

c. One of the cross-type sensors operates constantly at "normal precision" with any lens f5.6 or faster. If a lens is slower than f5.6, this array is switched off. This is not a function of light levels, but a switch function executed when the camera interrogates the lens for its maximum aperture. If a Canon teleconverter is attached, the teleconverter also reports its own factor and the camera takes that into account as well.

d. The other cross-type sensor is the "high precision" sensor, which is switched on when a lens of f2.8 or faster is attached. The vertically oriented array of this cross-type sensor apparently works in conjunction with the vertically oriented array of the "normal precision" cross-type sensor. It's not completely clear to me if this actually enhances precision, speed, or both, nor how the additional data is processed.

e. The horizontally oriented array of the "high precision" cross-type sensor is unique because the pixel segments are set farther apart than any of the other arrays to provide a significantly longer baseline. This definitely increases precision for vertically aligned subject details. It's not clear, though, how the camera integrates this data with the data sent by the horizontally oriented "normal precision" array.

Which brings us to one of the earlier questions. Which method is best to focus. Center point and recompose, all point, or select the closest point manually to what we want in focus without recomposing.

If the center point is the most accurate with 2.8 lenses, looks like that would be preferred. But then you get into the angle error like was previously explained.

oldsouth wrote:
Which brings us to one of the earlier questions. Which method is best to focus. Center point and recompose, all point, or select the closest point manually to what we want in focus without recomposing.

If the center point is the most accurate with 2.8 lenses, looks like that would be preferred. But then you get into the angle error like was previously explained.

I and most other old guys have always been using FandRC since forever without a problem on film cameras--or, I should say, with manual focus cameras.

However, we rarely attempted to shoot as much razor-thin depth of field work as you youngsters these days. Whenever I did something like that, I was normally using a plain groundglass focusing screen, setting my composition first and focusing directly on the part of the subject that I wanted sharp--no FandRC. Obviously it was manual focusing.

I believe this issue has raised its head primarily because people are trying to do the same thing with autofocusing, but IMO if you're working with razor-thin depth of field and want the zone of sharpness to fall on a specific plane (like midway the forward eye--differences of only a few millimeters when body sway can be measured in two or three centimeters), you really need to be operating manually and not FandRC. Even body sway (yours or the subject's) is enough to throw off such narrow depth of field, so you'd have to watch closely to shoot at the moment it fell right where you wanted it.

This is a problem similar to that of a target pistol or rifle shooter firing "offhand" (without physical support). It's physically impossible for a human to remain absolutely still without support, so the shooter learns to watch the sights as his natural body sway moves the point of aim over the bullsey, then time his trigger squeeze to fire as the aimpoint is right over the bullseye.

In the same way, when shooting for a razor-thin depth of field, photographers will have to compose, focus manually, and then watch how the zone of focus moves as he and the subject sway natrually, timing the shutter release for when the zone of focus is precisely where the photographer wants it to be.

add to that the AF cutting off with lenses over f5.6 (such as with a TC) when often the camera can still AF or at least make the attempt. I think I once heard of a fix involving blocking one of the contacts though I don't know where I saw it.

mtl337 wrote:
If the precision AF is activated by a switch, wouldn't it be nice if there was a hack that would 'trick' my 20D into activating the cross-type sensor for my f/5.6 or larger aperture lenses ....?

Why does it have to be f/2.8 lenses (besides the effort on Canon's part to force me to purchase $$$ glass)?

When that happens, though, the camera is using an erroneously shallow depth of focus figure for its calcuations. There would certainly be a slow-down of focusing at the very least because it will be trying to determine a higher degree of precision at a lower level of illumination that the system was designed for. If the lens starts out greatly defocused, it will probably do the in/out rack, fail to find the focus point because it's calculating too slowly, and give up.

oldsouth wrote:
Which brings us to one of the earlier questions. Which method is best to focus. Center point and recompose, all point, or select the closest point manually to what we want in focus without recomposing.

If the center point is the most accurate with 2.8 lenses, looks like that would be preferred. But then you get into the angle error like was previously explained.

WOW! Thanks for all the excellent contributions to this incredible thread.
Anyhow, as to the best mode, if the 20D had a split-prism I might be all for manual focus, but I'm just not convinced it is worth going with the third party screen either.
However, having read this thread and experimented with what the sensors respond to horizontally or vertically, I am much more proficient with the individual focus point selection now.

First of all many thanks for the article very illuminating. I wish to report my experience, too.

I shoot in TV shows, so low light conditions, fast actors movements, very tight timings for focusing.
From this you need to work at least at 800 ISO, autofocus (no time to manual focus at all), focusing button separated from shooting button (I don't remember wich custom function is) and center focusing point manually selected, no time to let the camera decide what to focus on.

I started last season with my 17-85 IS f/4-5.6 and I achived average results due to the lens too slow. Thanks to some lucky strikes and the IS I got some good pictures.
Anyway the focusing precision of this lens was good: 80% of picture had the focus where I wanted.

This summer I decided to invest some money and buy the 70-200 f/2.8 L IS to have a faster lens, pro picture quality and sharpness.
I bought it one month ago and I'm very disappointed with it!!!

I've made a lot of shots of friends of mine to test the lens and I must say that its precision on focusing is far from being satisfactory! The proportion is the opposite now: the 80% of shots are out of focus because the camera focuses 5 to 15 cm beyond or before the point I aim.
The condition of the test shots are much more easy than those of studios!!! I've tryed it outside, in daylight conditions, and even with sitting subjects! Of course it's always at f/2.8, otherwise there is no point in such a lens!
Having seen this, I tried some tests with the 50 f/1.8 and I got the same results (I didn't noticed this before with it because I never shoot that wide open).

Now I understand why! It seems that the "high precision" sensor that turns on when you put on the 2.8 lens is a lot less precise than the normal one!

Now I'm desperate! I'm starting to work for the new season this week and don't know how to solve this problem!
Any suggestion? Many thanks.

I would suggest giving the lens and camera to Canon to take a look at it. Most people find the 70-200 2.8L IS very satisfactory, as do I. I had to send the lens and 20D to Canon for focus adjust when brand new. They did a great job. BTW they went to Irvine.

howgus wrote:
I would suggest giving the lens and camera to Canon to take a look at it. Most people find the 70-200 2.8L IS very satisfactory, as do I. I had to send the lens and 20D to Canon for focus adjust when brand new. They did a great job. BTW they went to Irvine.

So they solved a similar problem? mmmh , I think I'll do the same... I live in a big city, so I think canon assistance is close at home. I'll keep you informed...
Thanks!

My understanding is that the 1-series currently has higher focus sensitivity at f2.8. Is it technically feasible that this feature could be extended to f4? IE for use with my 500f4.

AF is my single biggest problem with my 20D. I am planning to upgrade to the 1D-mk2 replacement when that is available, and primarily for better AF. I am hoping it will have improved AF even by 1-series standards. In my situation, AF is the single most important upgrade.

Update on my 70-200 f/2.8 IS: finally I got to give the lens to canon service to tune autofocus. I'm getting it back in the afternoon.
This week end big tests and monday I'll report you any changes...
Let's hope...!!!